Most of the brain and spinal cord development of the American opossum, particularly that of the lumbosacral cord, takes place after birth. The young are born about 12 days after conception and are carried in the mother's external pouch for 90 days or more. In the latter location they are accessible for both direct observation of developing motor activity and experimental manipulation, with little or no maternal trauma. Since previous work reveals that opossum spinal connections are typically mammalian, i.e., similar to those of more commonly used laboratory animals, we intend to: (1) continue our analysis of lumbrosacral supraspinal, propriospinal and dorsal root projections in the adult animal as a reference for interpreting the results of development studies; (2) study the development of such connections and their possible correlation with the onset and maturation of motor activity; and (3) study the development of cerebellar input to the brainstem centers which project to the lumbosacral cord. The lumbosacral enlargement is the focus of attention because of its marked immaturity at birth in the opossum and because undercutting of its inputs for experimental work is compatible with life in the developing animal.